Hardenable stainless steel alloys



June 27, 1961 NICKEL-Z w. o. BINDER ET AL 2,990,275 HARDENABLE STAINLESS STEEL ALLOYS Filed Sept. 19, 1958 (IOBALT-% INVENTORS WILLIAM O. BINDER By CHARLES M. BROWN 2224M}. m

ATTORNEY 2,990,275 HARDENABLE STAINLESS STEEL ALLOYS William 0. Binder and Charles M. Brown, Lewiston,

N.Y., assignors to Union Carbide Corporation, a corporation of New York Filed Sept. 19, 1958, 'Ser. No. 762,161 12 Claims. (Cl. 75-126) This invention relates to austenitic stainless steel alloys, and more particularly to transformable heat-hardenable austenitic stainless steel alloys.

-In the present state of the art, numerous hardenable steel alloys are produced and readily available. The important feature of these hardenable steel alloys which ensures their commercial acceptance, is that such steels can be initially annealed to produce an unstable austenitic structure, in which condition the steel is workable in almost any manner to produce a wide variety of forms and shapes. By subsequent heat-treatment, it is then possible to strengthen these alloys either by transformation to martensite or by precipitation of an intermetallic compound, or by a combination of both mechanisms. Articles of complex shape are thus capable of being cast directly or various types of structural forms, such as bars, rods, and sheets may be wrought from the austenitic steel since in the pre-hardened condition the metal is relatively soft and capable of being worked. After heat-treatment at sub-Zero and at elevated temperatures for specific lengths of time, hardness, yield strength, and tensile strength values are materially heightened. The degree of increase in physical properties depends largely upon the particular alloying constituents making up the alloy and the heat-treatment employed.

Commercial precipitation-hardened steels, and specifically those having as a primary added constituent chromium and as a secondary added constituent nickel, exhibit, in the hardened condition, Rockwell C values ranging from about 40 to 45 and tensile strength values on the order of 200,000 pounds per square inch. In order to ensure these values, the fabrication of hardenable steels almost invariably requires adherence to a very narrow alloying range of the primary and secondary constituents and a critical four-stage heat-treatment which includes a preliminary heat-treatment at elevated temperatures, an extended low-temperature treatment (approximately 8 hours), and another treatment at high temperature to produce the hardening effect. Compliance with the particular heat-treatment specified for a specific alloy is mandatory at the risk of considerable loss of hardness and strength.

Although all commercial hardenable steels are characterized by acceptable hardness, yield, and tensile strength values in the treated condition, such steels at times may exhibit extremely low impact strength. For example, specimens of precipitation hardenable 17% chromium-7% nickel stainless steels, when subjected to Izod impact tests, have exhibited values ranging from 4.0 to 13.0 ft-lb. In addition, as previously stated, any alteration in the heat-treatment of certain of these steels, as for instance reducing the low-temperature soaking period from the 8-hour period to two hours results in drastic losses in hardness, yield, and tensile strength values. Furthermore, the optimum quantity or ratio of certain constituents which must be present in these hardenable steels is generally so critical that severe losses in physical properties occur if the constituents are not present in the alloys in the precise quantities or amounts required. If one of the important components is nickel, for example, the percentage required to produce hardenability may be so close to the lower nickel range of 18 chromium-8 nickel steel that the danger of forming a stable rather than a transformable austenitic structure is 2,990,275 Patented June 27, 1 961 2 i a real possibility. If this possibility occurs, a hardenable steel is thereby produced.

Alloys falling within the scope of the present invention are based on a novel combination of metallic constituents and represent a distinct improvement over most hardenable alloys presently in use by virtue of overcomiing one or more of the disadvantages listed above.

It is an objective of the present invention, therefore, to provide hardenable steels of novel composition, whereby a relatively broad range of essential constituents may be employed, and wherein the danger of producing a stable austenitic structure is reduced.

Another object of the invention is the provision of novel hardenable stainless steel alloys which are characterized by high impact values, and which retain their physical properties to a considerable degree at elevated temperatures.

Still another object of the present invention is to provide a broad range of alloy compositions that are susceptible to an economical heat-treatment procedure and are simple to the extent that they may be varied with facility, thereby eliecting a saving in time with but little sacrifice of hardness and strength values.

The above objects are realized by the discovery that the inclusion of cobalt in hardenable steel compositions serves to reduce strict adherence to narrow compositional ranges without reducing the value or effectiveness of the hardenable steel alloy.

According to this invention, a hardenable stainless steel alloy is provided which consists essentially of 15.0 to 18.5 percent by weight chromium, 0.5 to 22.5 percent by weight cobalt, up to 6 percent by weight nickel, up to 0.2 percent by Weight carbon, up to 1.50 percent by weight manganese, up to 0.75 percent by weight silicon, up to approximately 0.15 percent by weight nitrogen, and the balance substantially iron. The aggregate amount of the cobalt and nickel content should be sufficient to impart a substantially austenitic structure at a temperature of above approximately 950 C., which amount is also partially dependent upon the carbon, nitrogen, and chromium content of the steel.

The preferred range of constituents of the instant alloys, however, is from 17.5 to 18.5 percent by weight chromium, 0.5 to 5.0 percent by weight cobalt, 4.5 to 5.5 percent by weight nickel, 0.03 to 0.10 percent by Weight carbon, up to 1.5 percent by weight manganese, up to 0.4 percent by weight silicon, and up to 0.15 percent by weight nitrogen, the balance iron and incidental impurities.

The relationship between the amounts of cobalt and nickel included in an 18.0 percent by weight chromium, 0.03 percent by weight carbon-steel alloy, embodying the invention, may be expressed experimentally in terms of the attached graph. The graph indicates the broad range within which the nickel and cobalt may be varied with respect to each other. This broad range is indicated by the shaded area of the graph, the boundaries of which are defined by the formulas x=0.5, y=0, where y=percent nickel and x=percent cobalt.

The above formulas will be varied somewhat where higher carbon and/or nitrogen levels are present, that is to say, the area established by the curves of the graph will be correspondingly narrower. Lower chromium levels will also tend to narrow the area defined by the curves.

Generally speaking, however, the complete elimination of nickel from the alloys of this invention is possible when cobalt is present in the range of between 10 to 22.5 percent by weight.

.The steel alloys of this invention represent a new group of hardenable alloys which in addition to having good hardness properties and strength in a heat-treated condition, are further characterized by high impact values, and less sensitivity to heat-treatment. They also permit a broader range of alloying consittuents than is ordinarily permissible with other steel alloys capable of being hardened by heat-treatment.

The term hardenable as used in connection with the stainless steel compositions of this invention refers to the alteration in the structure of the steel as a result of heat-treatment whereby strength and hardness are increased due to a transformation phenomena and/ or the precipitation of intermetallic compounds. Since the cobalt and nickel ranges are relatively broad, considerable variation is possible in the preparation of the transformable steel alloys of this invention. This feature is important commercially since it enables alloys to be prepared which possess the properties enumerated above to a high degree without entailing the heretofore necessary requirement of having a narrow range of the main alloying constituents. The possibility, therefore, of obtaining a martensitic structure or forming or retaining an extremely stable austenitic structure upon quenching after the initial annealing temperature is reduced due to this freedom from narrow range restrictions. With cobalt present, the criticality of the nickel range is lessened.

The alloys of this invention were tested in the following manner in order to compile tensile or hardenability data. Standard 0.05-inch sheet specimens were subjected to a three-stage heat-treatment in order to transform the unstabilized austenitic structure of the steel alloys to a precipitation-hardened condition. Hardness data were compiled during each of the three stages of heating and the data appearing in Table I below resulted. The three stages of heating consisted of /2 hour at 1075 C., followed by heating for two hours at 78 C., followed by heating for one hour at 475 C. It was found that this heat-treatment resulted in the desired hardenable steels. This contrasted sharply with the heat-treatment heretofore found necessary for other commercially available hardenable steels.

Generally speaking, the latter require an eight hour heat soaking at about minus 23 C. It should be understood, however, that the above three-stage heat-treatment is employed merely to illustrate that the alloys of the invention may he hardened by a simple heat treatment. If desired, more complex heating methods may be employed to enhance the properties of the alloys even further.

In all of the sheet specimens of the alloys of this invention the manganese, silicon, and carbon contents were 1.0%, 0.4 and 0.03% respectively. The results of the test were as follows.

e hour heating at 1,075 C.

b 2 hours heating at 78 C.

e 1 hour heating at 475 C.

Table I confirms that the presence of as much as 22.5 percent cobalt in the alloys of this invention will still result in a desirable hardness. It is to be observed also that low hardness following the first heating stage, hereinafter referred to as the as-quenched condition, was obtainable, especially in the 15 to 22.5 percent cobalt range. In this range as aforementioned, no nickel need be present to derive the hardening elfect, although up to about 4 percent nickel may be optionally present. Such highcobalt low-nickel compositions may be age-hardened to Rockwell C values above 36 by heat-treatment. If desired, the low temperature treatment may be eliminated.

The following table indicates a similar series of tests, including tensile and elongation data, employing alloys containing a relatively low percentage of cobalt. These specimens contain chromium, cobalt, and nickel constituents within the preferred percentage range, i.e., 17.5% to 18.5% chromium, 0.5% to 5.0% cobalt, and 4.5% to 5.5% nickel. In addition, 1.5% manganese and 0.4% silicon, and 0.03% carbon were included in the alloys, with the balance being iron. The results are tabulated below in Table II.

Table II Specimen N 0.

Cr Ni Co 1 a 2 b 3 s Room Temperature 900 F.

Stress Specimen No. at 0.2+

Ofiset, Tensile 2-Inch Gage Tensile 2-Iuch Gage p.s.i. Strength, Elong. Strength, Elong.

p.s.i. (Percent) p.s.i. (Percent) 105, 990 9.0 Not taken 177,000 13.0 Not taken 184,000 12.0 146, 900 l 8.0 153, 000 11.0 Not taken a hour heating at 1,075 O.

b 2 hours heating at 78 C.

a 1 hour heating at 475 C.

Table II substantiates that the cobalt-containing alloys of this invention may be hardened by a comparatively simple heat-treatment. The fourth specimen (10) did not contain cobalt. It was noted that very little variation in hardness occurred in the latter specimen during each of the three stages of heat-treatment. In addition, the tensile strength is considerably lower than those of the cobalt containing alloys.

Specimen 9 was subjected to high temperature tensile and elongation tests. The results of these above-indicated tests showed that a considerable degree of strength and ductility were retained in the hardened condition even at elevated temperatures.

Four separate /s-inch thick impact samples of specimen 9, which had been given the three-stage heat-treatment previously described, were subjected to Izod impact tests to determine to what degree the alloy of the invention resists impact. An average value of 50 ft.-lbs. was achieved.

Additional benefits may be derived by modifying the chromium-cobalt-nickel alloy composition of this invention with any one of or combination of the following metallic constituents: aluminum, copper, molybdenum, columbium, titanium, zirconium, vanadium, and tantalum. These eight elements may be presented up to the following maximum percentage levels:

Aluminum Up to 1.5 Copper Up to 3.0% Molybdenum Up to 3.0% Columbium Up to 1.0% Titanium Up to 1.0% Zirconium Up to 1.0% Vanadium Up to 1.0% Tantalum Up to 1.0%

A series of alloys was prepared in which the effect of the above constituents was studied specifically in connection with hardenability and the increase of tensile strength following heat-treatment. The following data 6 0.04% by weight nitrogen, the balance iron and incidental impurities, the aggregate of the cobalt and nickel being such as to insure a substantially hardenable and transformable austenite at above approximately 950 C.

were complled. 5 4. A hardenable, transformable, austenitic stainless Table III Specimen No. Cr Ni Co Al Cu Mo Cb Mn Si O HardnessRoom Temperature, Room Temperature Rockwell B or 0 Stress at Specimen No. 0.2%, p.s.i.

Tensile E1ong., 1 2 3 Strength, Percent R0 34. 0 R 36. 0 95, 000 189, 000 30. 0 Rs 33.0 Rs 34.6 103, 100 183, 800 20.0 RC 37. 0 R0 42. 0 161, 700 194, 800 13.0 R 20. 0 R0 40. 0 166, 500 182,100 11. 5 R 40. 0 RC 44. 0 159, 500 217, 000 15. 0 R0 33. 0 Re 40. 0 135, 700 186, 200 13. 5 R0 29. 0 R0 39. 0 155, 000 189, 300 9. 0 R 39. 3 Re 44. 2 172, 900 213, 000 14. 0 R0 34. 2 Be 42.0 150, 700 191, 000 7. 0 R 37.0 R0 45.0 Rt; 27. 6 R0 36. 4 149, 400 169, 000 11. 0 R0 33. 0 R0 45. 0

n hour heating at 1075 C. b 2 hours heating at 78 C. c 1 hour heating at 475 C.

The above data indicates that the low hardness values in the as-quenched condition may be altered to much higher hardnesses, and good tensile strength may be achieved by the modified chromium-cobalt-nickel alloys of the invention. High strength and hardness are achieved with the addition of 0.5% aluminum and 0.5% molybdenum. In addition, although in the cobalt, 3% copper heat no nickel was present, a steel was produced which, although low hardness characterized the as-quenched sample, proved quite hardenable after a simple heat-treatment.

What is claimed is:

1. A hardenable transformable, austenitic stainless steel alloy consisting essentially of about 15.0% to 18.5% by Weight chromium, about 10% to 22.5% by weight cobalt, up to about 0.2% by weight carbon, up to about 1.5% by weight manganese, up to about 0.75% by weight silicon, up to about 0.15% by weight nitrogen, and the balance substantially iron.

2. An alloy as claimed in claim 1 in which up to about 4 percent by weight of nickel is present, the cobalt and nickel constituents being present in a relationship such as to insure a substantially hardenable and transformable austenite at above approximately 950 C. and expressed by the area defined by the curves defined by the formulas x=0.5, y=0, wherein y denotes the percent nickel and x denotes the percent cobalt.

3. A hardenable, transformable, austenitic stainless steel alloy consisting of about 18.0% by weight chromium, about 4.0% by Weight nickel, about 5.0% by weight cobalt, about 0.03% by weight carbon, about 1.5% by weight manganese, about 0.4% by weight silicon, about steel alloy consisting of about 18.5% by weight chromium, about 5.0% by weight nickel, about 2.0% by weight cobalt, about 0.5% by weight aluminum, about 0.5% by Weight molybdenum, about 1.5% by weight manganese, about 0.4% by weight silicon, about 0.04% by weight nitrogen, about 0.07% by weight carbon, and the balance iron and incidental impurities, the aggregate of the cobalt and nickel being such as to insure a substantially hardenable and transformable austenite at above approximately 950 C.

5. A hardenable, transformable, austenitic stainless steel alloy consisting of about 18.3% by weight chromium, about 15.0% by Weight cobalt, about 3.0% by weight copper, about 1.5% by weight manganese, about 0.4% by weight silicon, about 0.03% by weight carbon, about 0.04% by weight nitrogen, and the balance iron and incidental impurities.

6. A hardenable, transformable, austenitic stainless steel alloy consisting essentially of about 15.0 percent to about 18.5 percent by weight chromium, about 0.5 percent to about 22.5 percent by weight cobalt, up to about 6 percent by weight nickel, up to about 0.2 percent by weight carbon, up to about 1.5 percent by weight manganese, up to about 0.75 percent by weight silicon, up to about 0.15 percent by weight nitrogen, and the balance substantially iron and incidental impurities, the cobalt and nickel constituents being present in a relationship such as to insure a substantially hardenable and transforrnable austenite at above approximately 950 C. and falling within the area defined by the curves defined by the formulas x=0.5, y=0, wherein y denotes the percent nickel and x denotes the percent cobalt.

7. Altardenable, transformable, austenitic stainless steel alloy consisting essentially of about 17.5 percent to 18.5 percent'by Weight chromium, about 0.5 percent to 5.0 percent by weight cobalt, about 4.5 percent to 5.5 percent by weight nickel, about 0.03 percent to 0.10 percent by weight carbon, up to about 1.5 percent by weight manganese, up to about 0.4 percent by weight silicon, up to about 0.15 percent by weight nitrogen, and the balance substantially iron, the cobalt and nickel constituents being present in a relationship such as to insure a substantially hardenable and transformable austenite at above approximately 950 C. and falling within the area defined by the curves defined by the formulas x=0.5, y=0, wherein y denotes the percent nickel and x denotes the percent cobalt.

8. A hardenable, transformable, austenitic stainless steel alloy consisting essentially of about 18 percent by weight chromium, about 0.5 percent to 22.5 percent by weight cobalt, up to about 6 percent by weight nickel, about 0.03 percent by Weight carbon, up to about 1.5 percent by weight manganese, up to about 0.75 percent by weight silicon, up to about 0.05 percent by weight nitrogen, and the balance substantially iron, the cobalt and nickel constituents being present in a relationship expressed by the area defined by the curves defined by the formulas x=0.5, y=0, wherein y denotes the percent nickel and x denotes the percent cobalt.

9. A hardenable, transformable, austenitic stainless steel alloy consisting essentially of about 15.0 percent to 18.5 percent by weight chromium, about 0.5 percent to 22.5 percent by weight cobalt, up to about 6 percent by weight nickel, up to about 0.2 percent by weight carbon, up to about 1.5 percent by weight manganese, up to about 0.75 percent by weight silicon, up to about 0.15 percent by weight nitrogen, up to about 1.5 percent by weight aluminum, up to about 3.0 percent by weight copper, up to about 3 percent by weight molybdenum, up to about 1.0 percent by weight columbium, up to about 1.0 percent by weight titanium, and the balance substantially iron, the cobalt and nickel constituents being present in a relationship such as to insure a substantially hardenable and transformable austenite at above approximately 950 C. and falling within the area defined by the curves defined by the formulas x=0.5, y=0, wherein y denotes the percent nickel and x denotes the percent cobalt.

10. A hardenable, transformable, austenitic stainless steel alloy consisting essentially of about 17.5 percent to 18.5 percent by weight chromium, about 0.5 percent to 5.0 percent by weight cobalt, about 4.5 percent to 5.5 percent by weight nickel, up to about 0.10 percent by weight carbon, up to about 1.5 percent by weight manganese, up to about 0.4 percent by weight silicon, up to about 0.15 percent by weight nitrogen, up to about 1.5 percent by Weight aluminum, up to about 3.0 percent by weight copper, up to about 3.0 percent by weight molybdenum, up to about 1.0 percent by weight columbium, up to about 1.0 percent by weight titanium, and the balance substantially iron, the cobalt and nickel constituents being present in a relationship such as to insure a substantially hardenable and transformable austenite at above approximately 950 C. and falling within the area defined by the curves defined by the formulas x=0.5, y=0, wherein y denotes the percent nickel and x denotes the percent cobalt.

11. A hardened, transformed, austenitic stainless steel alloy characterized by Rockwell C hardness value of at least 36 and consisting essentially of about 15 .0 percent to about 18.5 percent by Weight chromium, about 0.5 percent to about 22.5 percent by weight cobalt, up to about 6 percent by weight nickel, up to about 0.2 percent by weight carbon, up to about 1.5 percent by weight manganese, up to about 0.75 percent by weight silicon, up to about 0.15 percent by weight nitrogen, and the balance substantially iron and incidental impurities, the cobalt and nickel constituents being present in a relationship falling within the area defined by the curves defined by the formulas x=0.5, y=0, wherein y denotes the percent nickel and x denotes the percent cobalt.

12. A hardened, transformed, austenitic stainless steel alloy characterized by a Rockwell C hardness value of at least 36 and consisting essentially of about 15 .0 percent to 18.5 percent by weight chromium, about 0.5 percent to 22.5 percent by weight cobalt, up to about 6 percent by weight nickel, up to about 0.2 percent by weight carbon, up to about 1.5 percent by weight manganese, up to about 0.75 percent by weightsilicon, up to about 0.15 percent by weight nitrogen, up to about 1.5 percent by weight aluminum, up to about 3.0 percent by weight copper, up to about 3 percent by weight molybdenum, up to about 1.0 percent by weight columbium, up to about 1.0 percent by weight titanium, and the balance substantially iron, the cobalt and nickel constituents being present in a relationship falling within the area defined by the curves defined by the formulas x=0.5, y=0, wherein y denotes the percent nickel and x denotes the percent cobalt.

References Cited in the file of this patent UNITED STATES PATENTS 2,505,764 Goller May 2, 1950 2,536,034 Clarke Jan. 2, 1951 2,537,477 Mohling et al Jan. 9, 1951 2,590,835 Kirkby et al Apr. 1, 1952 2,892,702 Walton et a1 June 30, 1959 2,894,867 Smith July 14, 1959 2,903,386 Waxweiler Sept. 8, 1959 FOREIGN PATENTS 598,144 Great Britain Feb. 11, 1948 669,579 Great Britain Apr. 2, 1952 730,272 Great Britain May 18, 1955 OTHER REFERENCES Freeman et 211.: ASTM Symposium on Materials for Gas Turbines, 1946, pages 52-79. Published by the American Society for Testing Materials, Philadelphia, Pa. 

9. A HARDENABLE, TRANSFORMABLE, AUSTENTIC STAINLESS STEEL ALLOY CONSISTING ESSENTIALLY OF ABOUT 15.0 PERCENT TO 18.5 PERCENT BY WEIGHT CHROMIUM, ABOUT 0.5 PERCENT TO 22.5 PERCENT BY WEIGHT COBALT, UP TO ABOUT 6 PERCENT BY WEIGHT NICKEL, UP TO ABOUT 0.2 PERCENT BY WEIGHT CARBON, UP TO ABOUT 1.5 PERCENT BY WEIGHT MANGANESE, UP TO ABOUT 0.75 PERCENT BY WEIGHT SILICON, UP TO ABOUT 0.15 PERCENT BY WEIGHT NITROGEN, UP TO ABOUT 1.5 PERCENT BY WEIGHT ALUMINUM, UP TO ABOUT 3.0 PERCENT BY WEIGHT COPPER, UP TO ABOUT 3 PERCENT BY WEIGHT MOLYBDENUM, UP TO ABOUT 1.0 PERCENT BY WEIGHT COLUMBIUM, UP TO ABOUT 1.0 PERCENT BY WEIGHT TITANIUM, AND THE BALANCE SUBSTANTIALLY IRON, THE COBALT AND NICKEL CONSTITUENTS BEING PRESENT IN A RELATIONSHIP SUCH AS TO INSURE A SUBSTANTIALLY HARDENABLE AND TRANSFORMABLE AUSTENITE AT ABOVE APPROXIMATELY 950*C. AND FALLING WITHIN THE AREA DEFINED BY THE CURVES DEFINED BY THE FORMULAS 